Antenna arrays are already known which are intended for concentrating the energy received from a mobile via multiple paths. To do this, it is preferable for the sensors to be mutually decorrelated. This decorrelation can be obtained by various processes. For example, the antennas can be designed to receive signals having different polarizations. The most frequent solution consists however in using antennas which are spaced sufficiently far apart. If the multiple paths are distributed in a substantially omnidirectional manner around the antenna array, a spacing of the sensors of the order of half the wavelength is sufficient to ensure decorrelation. However, the directions of arrival of the various multiple paths are generally distributed within a cone of relatively small aperture. Signal decorrelation therefore requires that the mutual spacing of the sensors be made so as greatly to exceed half the wavelength and so as in general to be greater than one wavelength.
The antenna array must also make it possible, by processing the signals received from a mobile, to locate it directionally so as subsequently to allow transmission to the mobile with concentration of energy towards the latter. The location processing mentioned above can be "explicit" and implement, for example, a process of the type described in the article by ANDERSON S., MILLNERT M., VIBERG M., WAHLBERG B., "An adaptive array for mobile communication systems", IEEE Transactions on Vehicular Technology, Vol. 40, No. 1, February 1991, pages 230-236. The location processing can also be "implicit" and implement, for example, a process of the type described in French Patent Application No. 96 13597.
The antenna arrays used at the present time do not make it possible simultaneously to carry out the two functions of location and diversity of reception, making it possible to improve reception, by virtue of the processing of mutually decorrelated signals.
This is because, at present, the same antenna array is generally used both for reception and transmission. Satisfactory location processing is possible only if the array is devoid of ambiguity, that is to say of side lobes whose amplitude is comparable to the amplitude of the main lobe. For more details regarding the concept of array ambiguity, useful reference may be made to the Thesis by A. FLIELLER, "Mise en oeuvre des methodes a haute resolution en traitement d'antenne: autocalibration robuste et geometrie des reseaux" [Implementation of high-resolution methods in antenna processing: robust autocalibration and array geometry], Thesis from the Universite de Paris-Sud, UFR Scientifique d'Orsay, Jun. 16, 1995.
The arrays which are most widely employed at the present time are arrays of the linear or circular type with equidistributed sensors. It is known that, in order to avoid ambiguity in this type of array, the sensors must be spaced about half a wavelength apart. This may lead to a very large number of sensors if the distance required to ensure decorrelation between at least two sensors is very large relative to the wavelength. But, on the grounds of cost and bulkiness, it is sought to limit the number of sensors. This known type of array therefore has drawbacks.
In a context other than that of radiocommunications, some authors have suggested the use of so-called sparse antenna arrays, linear arrays whose antennas are arranged in an irregular manner. However, even though the decorrelation between the sensors of such arrays is greater, for an equal number of sensors, than the decorrelation of an array with equidistributed sensors, it is generally insufficient for current propagation conditions. Moreover, arrays of the "sparse antennas" type are very sensitive to imperfections resulting from the positioning and calibration of the sensors.
The aim of the present invention is to alleviate the aforementioned drawbacks.